Technology, which utilizes hierarchized storage in units of pages for a more efficient storage system, is known (Patent Literature 1). This prior art defines multiple storage tiers in a logical pool, and allocates logical volumes to any tier of the multiple tiers on the basis of the performance of the individual logical volumes.
The storage system creates a virtual logical volume, and allocates this virtual logical volume to a host computer. The host computer performs either a write access or a read access with respect to a logical storage area (also called a logical page) comprising the virtual logical volume. When the host computer first accesses a logical page targeted for write access, a real storage area (also called a physical page) in the pool is associated with this logical page. Write data from the host computer is written to this physical page.
In a case where the host computer reads data from the virtual logical volume, the physical page storing the read-target data is identified based on the corresponding relationship between the logical page and the physical page. The storage system reads the read-target data from the identified physical page and sends this data to the host computer.
The association between a logical page and a physical page changes regularly in accordance with the amount of I/O (Input/Output) to the logical page. This change in the association is called page reallocation. The storage system monitors I/Os from the host computer to each logical volume, and stores the I/O load of each logical page. An I/O load threshold is defined beforehand for each storage tier in the pool. In a case where the I/O load of a certain logical page exceeds the threshold of the current allocation-destination storage tier, the storage system associates the data of this logical page with a physical page in a different storage tier. Generally speaking, as the I/O load of a logical page becomes larger, the data of this logical page is stored in a physical page of a higher performance storage tier. As the I/O load of the logical page becomes smaller, the data of this logical page is stored in a physical page of a lower performance storage tier.